Anthony Shannon, who has been charged with theft, claims he is an "activist not a criminal" and was compelled to liberate the sea creature over fears her welfare was being endangered by inadequate tank conditions.

But the 38-year-old now faces charges over stealing the fish and taking her to his home, where she was kept in a tank before being tracked down and returned to the aquarium by authorities.

Image:The shark theft was captured on CCTV

Miss Helen was reported to be one of around 25 sharks being kept in large pools at Mr Shannon's home, along with an unknown number of crabs.

The accused told a reporter that the theft was "regretful", but suggested he could justify such behaviour if it was an "emergency" for the sea creature concerned.

"I have a mission," he said.

"It's not a mission to steal for profit, it's a mission just to give the fish a better health."

Police chief Joseph Salvaggio said Shannon had "a lot of different marine animals" in his home and "very much knew what he was doing".

Staff at the San Antonio Aquarium denied accusations that animals were kept in inadequate conditions and said extensive tests were carried out on the water every day.

"His claim is they were sick and dying," Ammon Covino, the aquarium's founder, said. "We have thousands of animals and they are all healthy."

Mr Covino said he did "not sleep the whole night" after the shark theft and was kept awake with excitement following Miss Helen being returned in a healthy condition a few days later.

"You have to have a pet, really, to appreicate the love that animals and people have together, to appreciate the symbiotic relationship," he said.

"We know dogs will sleep on the end of your bed, will respond when you're sad. Fish do the same thing."